It's fair enough if you ask me. The attitude of some pax is vile and aggressive as if it is somehow their fault and they personally caused the delay for their own gain. Sometimes a quick none SOP message to remind passengers of a few things is required, particularly on weekend flights. Doing things high profile is usually good if something does happen and that way nobody can say the crew 'did nothing' in my opinion.
In this instance CC5 will likely be at the rear door/cabin and could have witnessed some inappropriate behaviour. And if they didn't then a quick PA to keep things real is usually a good thing and gets the rest of the passengers 'on side' as it were.
The fifth crew member was likely a trainer or there was crew training, assessments taking place or it was a crew manager who will fly a few times a month to keep current/in touch with reality.
A successful outcome was had by the looks of it. I only hope that pax didn't think it was too patronising or heavy handed.